Heat vegetable oil in a large pot placed over medium high heat. Brown chicken, until it shows some colour, and then remove from pan. Chicken doesn't need to be cooked all the way through.
Add butter and allow to melt and foam. Then add onion, garlic, ginger and lemongrass. Sauté for a few minutes until softened and fragrant, then season with salt.
Add red curry paste, stirring through the vegetables. Cook for 1-2 minutes until toasted and fragrant. Lower heat slightly before sprinkling over flour and stirring through, creating a roux.
Pour in chicken stock and coconut milk and scrape up any stuck bits at the bottom of the pan. Allow to simmer for a few minutes, stirring regularly, until the sauce has thickened. Add makrut lime leaves, sugar and fish sauce. Then add vegetables and chicken back in and stir everything well.
Transfer chicken mixture into an oven-safe dish. Top with puff pastry and crimp around the edges. Poke a few holes in the top to allow the steam to escape.
Preheat oven to 180ºC. Place pie in the oven for 20-30 minutes until pastry is golden and crisp and filling is bubbling. Serve immediately.